This invention relates to the formation of lubricant coatings on metal surfaces and is more particularly directed to an improved means for coating the interior surfaces of steel tubes with an adherent layer of zinc stearate.
It is well known that the endurance life of gun tubes can be appreciably increased by forcing a metal slug or a "swage" tool through the bore of the tube to induce residual stresses in the walls thereof which serve to partially counteract the stresses encountered during actual firing. In order to prevent seizing or galling by the swage tool, the bore surface of the gun tube is customarily coated with a layer of electrolytically deposited lead. While the physical characteristics of these coatings are generally satisfactory, the electroplating thereof on a production basis is extremely costly due to the need for frequent replacement of the plating anodes. In addition, the toxicity of the metal mandates the provision of special equipment and procedures for removing the health hazard to which the operating personnel are exposed. The total cost involved is even further increased by the considerable effort and time required to remove the lead coating from the gun tube prior to the firing thereof. These drawbacks have therefore prompted a search for other materials capable of providing the required degree of lubricity without any sacrifice in the adherence of the coating under the relatively high pressures generated by the forced passage of the swage tool through the bore of the gun tube.
A zinc stearate coating appeared to be an acceptable substitute for the lead in view of the excellent results achieved therewith in the drawing of thin-walled tubing. A particularly desirable characteristic of this material was the complete vaporization thereof at the temperatures produced during the swaging process. However, the application of the coating to the bore surfaces by the conventional procedure of dipping the tubes into the required treatment fluids was found to be impractical due to the cumbersome and expensive handling equipment required to transfer the relatively long and heavy tubes from one treatment fluid to the other. Another problem lay in the relatively complex cleaning and rinsing procedures required to prevent any intermingling and resulting contamination between the separate treatment fluids.